iwlwifi: mvm: fix RX SKB header size and align it properly

When receiving a frame, we currently pull in sizeof(*hdr) plus
some extra (crypto/snap), which is too much, most headers aren't
actually sizeof(*hdr) since that takes into account the 4-address
format but doesn't take into account QoS. As a result, a typical
frame will have 4 bytes of the payload in the SKB header already.

Fix this by calculating the correct header length, and now that
we have that, align the end of the SKB header to a multiple of 4
so that the IP header will be aligned properly when pulled in.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>